Protein kinases have been the subject of extensive study in the search for new therapeutic agents in various diseases, for example, cancer. Protein kinases are known to mediate intracellular signal transduction by effecting a phosphoryl transfer from a nucleoside triphosphate to a protein acceptor that is involved in a signaling pathway. There are a number of kinases and pathways through which extracellular and other stimuli cause a variety of cellular responses to occur inside the cell.
The polo-like kinase (PLK) family of serine/threonine kinases comprises at least four known members: PLK1, PLK2 (also known as Snk), PLK3 (also known as Fnk or Prk) and PLK4 (also known as Sak). PLK4 is the least understood and most divergent member of the PLK family. The N-terminal catalytic domain of PLK4 has a different substrate specificity from that of PLK1-3. PLK4 also has a divergent C-terminus comprising only a single polo-box sequence, not the tandem PB sequences in PLK1-3, that appears to act as a homodimerization domain rather than a localization domain (Lowery et al., (2005) Oncogene 24: 248-259).
PLK4 is known to be involved in the control of mitotic entry and exit, and a regulator of centrosome duplication (Habedanck et al. Nature Cell Biology 7: 1140-1146, 2005). PLK4 transcripts increase from S through M phase, and the protein is ubiquitylated and destroyed by the anaphase promoting complex (APC) (Hudson et al. Curr. Biol. 11: 441-446, 2001; Fode et al. Mol. Cell. Biol. 16: 4665-4672, 1996). PLK4 is required for late mitotic progression (Fode et al. PNAS. 91: 6388-6392, 1994; Hudson et al. Curr. Biol. 11: 441-446, 2001), cell survival and postgastrulation embryonic development (Hudson et al. Curr. Biol. 11: 441-446, 2001). PLK4 knockout mice are embryonic lethal (E7.5), with a marked increase in mitotic and apoptotic cells (Hudson et al. Curr. Biol. 11: 441-446, 2001). PLK4 is transcriptionally repressed by p53 (Li et al. Neoplasia 7: 312-323, 2005). This repression is likely mediated through the recruitment of histone deacetylase (HDAC) repressors and repression appears to contribute to p53-induced apoptosis (Li et al. Neoplasia 7: 312-323, 2005).
PLK4 has been reported to be overexpressed in colorectal tumors with expression reported as low in adjacent normal intestinal mucosa (Macmillian et al. Ann. Surg. Oncol. 8: 729-740, 2001). In addition, PLK4 mRNA has been reported to be overexpressed in some tumor cell lines (Hitoshi, et al., U.S. Patent Application No. US 2003/0027756). In addition, Applicants described overexpression of PLK4 in basal-like tumors in a co-pending U.S. Provisional Application No. 61,003,825, filed on Nov. 20, 2007 (the entire teachings of which are incorporated herein by reference).
Therefore, agents which inhibit a protein kinase, in particular PLK4, have the potential to treat cancer. There is a need for additional agents which can act as protein kinase inhibitors, in particular PLK4 inhibitors.